The present invention relates to a digital FM modulator, and more particularly to a digital FM modulator to be used for transmitting video signals and audio signals over a frequency modulated (FM) wave.
A digital FM modulator excelling in frequency stability consists of an analog-to-digital (A/D) converter, an adder and a direct digital synthesizer (DDS) as described, for instance in JP-A-62-266903. The A/D converter converts such analog modulation signals as video signals and audio signals into digital modulation signals. The adder adds the digital modulation signals to addition data for setting the center frequency of the FM wave, and supplies added digital signals. The DDS, comprising an accumulator and a read only memory (ROM), accumulates the added digital signals in every sampling period. The accumulated signal becomes an address signal to the ROM, from which a sine wave corresponding to the FM wave is outputted.
Such a digital FM modulator can set the center frequency of the FM wave as addition data, and can determine the frequency stability merely by the accuracy of a reference oscillator which determines the sampling period.
However, with this digital FM modulator, if the frequency, amplitude or the like of the modulation signals, which are the input signals, varies, data of the added digital signals will also vary because the addition data are the same. As a consequence, the average frequency of the FM wave would deviate from the center frequency of the FM wave, resulting in a deterioration in frequency stability. There would also arise the problem of interference with adjoining channels.